The present invention relates to a coated cemented carbide cutting tool insert particularly useful for turning operations in steels or stainless steels, and is especially suited for operations with high demands regarding toughness properties of the insert.
High performance cutting tools must nowadays possess high wear resistance, high toughness properties and good resistance to plastic deformation. Improved toughness behaviour of a cutting insert can be obtained by increasing the WC grain size and/or by raising the overall binder phase content, but such changes will simultaneously result in significant loss of the plastic deformation resistance.
Methods to improve the toughness behaviour by introducing thick essentially gamma phase-free and binder phase-enriched surface zone with a thickness of about 20-40 xcexcm on the inserts by a so-called xe2x80x9cgradient sinteringxe2x80x9d techniques have been known for some time e.g. U.S. Pat. No. 4,277,283, U.S. Pat. No. 4,497,874, U.S. Pat. No. 4,548,786, U.S. Pat. No. 4,610,931, U.S. Pat. No. 5,484,468, U.S. Pat. No. 5,549,980, U.S. Pat. No. 5,649,279, U.S. Pat. No.5,729,823. The characteristics of these patents are that the surface zone has a different composition than the bulk composition, and is depleted of gamma phase and binder phase enriched.
It has now surprisingly been found that by using a gamma phase consisting essentially of only TaC and TiC in addition to WC, by keeping the ratio between the elements Ta and Ti within specific limits, and having a highly W-alloyed binder phase, the toughness properties of the gradient sintered cutting inserts can be significantly improved without any loss of plastic deformation resistance.
A first aspect of the present invention provides a cutting tool insert for machining steel comprising a cemented carbide body comprising WC, 5-12 wt. % Co, 3-11 wt. % of cubic carbides of the metals Ta and Ti, and less than 0.1 wt. % of Nb where the ratio of Ta/Ti is 1.0-4.0, and the Co-binder phase is highly alloyed with W, having a CW-ratio of 0.75-0.95, the body also comprising a binder phase enriched and essentially gamma phase free surface zone with a thickness of 5-50 xcexcm; and a coating.
A second aspect of the present invention provides a method of making a coated cemented carbide body having a gamma phase-free and binder rich surface zone comprising the steps of:
(i) forming a powder mixture comprising WC, 5-12 wt. % Co, 3-11 wt. % cubic carbides of Ta and Ti, where the ratio of Ta/Ti is 1.0-4.0;
(ii) adding N in an amount of 0.6-2.0% of the weight of Ta and Ti;
(iii) milling and spray drying the mixture to form a powder material with the desired properties;
(iv) compacting and sintering the powder material at a temperature of 1300-1500xc2x0 C., in a controlled atmosphere of about 50 mbar followed by cooling, whereby a body having a binder phase enriched and essentially gamma phase free surface zone of 5-50 xcexcm in thickness is obtained;
(v) applying a pre-coating treatment to the body; and
(vi) applying a hard, wear resistant coating.